(1) Field of the Invention
The invention generally relates to a semiconductor wafer carrier and, more particularly to methods of improving the apparatus used in holding a semiconductor wafer during a chemical mechanical polishing (CMP) process.
(2) Description of Prior Art
Semiconductor fabrication often uses a combination of chemical and mechanical polishing to reduce the thickness and planarize a thin film coating on a wafer. Typically, the wafer is placed in a polishing head and makes contact with a rotating polishing pad having a slurry applied thereto. Often the polishing head holding the wafer also rotates making the planarization process more uniform.
FIG. 1 illustrates a cross section of the current art for the polishing process. The wafer 14 is held in place laterally by the extension ring 20. To facilitate thin film planarization, uniform pressure is applied mechanically from above to the carrier 18 holding the wafer 14 firmly against the polishing pad 12. To aid in maintaining uniform pressure to the wafer 14, a thin backing film 16 is usually attached to the carrier 18. The polishing table 10 and polishing pad 12 are rotated at a set speed, while often, the carrier 18, backing film 16, and wafer 14 rotate at a second set speed. During automated loading and unloading, the wafer is held onto the carrier by vacuum pressure via passages 22.
Using the current methods of CMP to polish a wafer, less material is removed from the edge of the wafer than from the center. This is due to a phenomenon known as "pad rebound" or "waving phenomenon" and results in non-functioning devices on the wafer edge. FIG. 2a shows a magnified cross section of the edge of the wafer 14, the polishing pad 12, the carrier 18, the backing film 16 and extension ring 20. When the wafer 14 is pressed downward onto the pad 12, a stress concentration 38 occurs just inside the outer edge of the extension ring 20 as the pad 12 is pressed against the extension ring 20 and wafer 30. This results in the pad 12 rebounding away from the extension ring 20 and wafer 14. This is illustrated by the exaggerated dip 39 in the pad 12. FIG. 2b shows graphically the result of the pad rebound phenomenon. The extension ring is typically 3 to 4 mm wide. A portion of the pad rebound (.about.3 to 4 mm from the edge of the extension ring 12) occurs under the extension ring (region 32). Because of the pad rebound, material removal rate at the interface 30 between the ring and wafer is approximately at a minimum. The material removal rate increases toward the center of the wafer 14, and becomes constant at .about.6 to 7 mm (region 37) inside the edge of the extension ring (2 to 4 mm from the edge of the wafer). Unfortunately, the edge of the wafer (region 34) has a higher material removal rate and is therefore unusable.
Other approaches attempt to address problems with pad rebound during polishing. U.S. Pat. No. 5,795,215 to Guthrie et al. teaches a method using different pressures applied to the carrier and extension ring. U.S. Pat. No. 5,876,273 to Yano et al teaches a method using a pressure-absorbing member between the carrier and extension ring. This member allows movement of the extension ring with respect to the carrier while maintaining uniform pressure on the wafer. Another embodiment has a circular plate surrounding the wafer. U.S. Pat. No. 5,785,584 to Marmillion et al teaches a method utilizing a raised section on the polishing pad. U.S. Pat. No. 5,635,083 to Breivogel et al teaches a method whereby an air pillow under the wafer holds it flat against the polishing pad. It also utilizes different pressures on the carrier and wear ring to minimize pad rebounding. U.S. Pat. No. 5,876,271 to Oliver teaches a method whereby slurry is applied to the wafer surface though a plurality of holes in the surface of the polishing pad. U.S. Pat. No. 5,851,140 to Barns et al. teaches a method using a flexible carrier plate providing an air pillow that maintains uniform pressure on the wafer during CMP.